High power semiconducting devices including, but not limited to, LEDs, laser diodes, HEMTs, solar cells, photoelectrochemical cells, and RF devices, are becoming increasingly important. While silicon has typically dominated many of these markets, increasingly nitrides, carbides, and oxides are becoming important electronic materials based on their superior properties. Nitrides have become the material of choice for LEDs and they are also rapidly expanding into high frequency and power devices. Silicon carbide and ZnO also are becoming important electronic materials.
In many cases the use of transfer techniques such as waferbonding are required for high power semiconducting devices using nitride, carbide and oxide epitaxial layers due to the lack of native substrates. Thin epitaxial layers are transferred onto a variety of substrates ranging from silicon to metal composites. Typically, a solder layer is used to bond the thin epitaxial layer to the underlying support substrate.
Methods and articles based on thick epitaxial layers, which do not require an underlying support substrate, can be formed inexpensively based on HVPE templates, such as co-pending U.S. patent application Ser. Nos. 12/221,304; 12/462,295; and 12/583,527, commonly assigned as the present application and herein incorporated by reference. These large area freestanding epitaxial chips are formed by a novel laser liftoff process, which does not crack the epitaxial layer even without wafer bonding steps. Using this approach, 30 micron thick epitaxial chips with sizes greater than 1 inch square have been formed. These large area freestanding epitaxial chips are potentially less expensive to manufacture than the more conventional prior art method of using large wafers bonded to non-native substrates with epitaxial growth followed by dicing and polishing. The freestanding epitaxial chips also exhibit reduced stresses, don't require polishing steps, which induce defects, and are flexible. In particular it has been disclosed that the emission spectra of devices grown on these large area freestanding epitaxial chips exhibit wider bandwidth and unique spectra from devices fabricated with the growth substrate attached in wafer form or on diced substrates. This appears to be due to the removal of stresses formed during the growth process.
It has been found that the flexible nature and nitride only nature of the freestanding epitaxial chip enables the use of high temperature materials not normally possible with semiconductor fabrication processes such as high temperature glasses and other materials typically used in incandescent light and arc lamp markets. These materials were developed over the last century to withstand not only the high flux levels created by these sources but also to meet the environmental requirements of everyday use. Low cost manufacturing processes for solid state lighting (SSL) have been sought with only limited success by utilizing organic low temperature materials which can withstand both the flux levels and environmental requirements of lighting. However, the low temperature processing requirement has limited the use of inorganic materials which form robust hermetic seals against the environment. Organic light emitting diodes were sought to be a low cost method of fabricating solid-state lighting. However, the organic materials have not held up to the operating and environmental conditions required in their use.
The intent of this invention is to disclose methods, materials, and applications in which high temperature processes and materials can be used to make devices for lighting, displays, solar cells, and electronics, which overcome the deficiencies, that organic and low temperature materials exhibit. The large area freestanding epitaxial layer disclosed is compatible with thick film materials and processes up to 700 degrees C. in air and higher in controlled atmospheres. This high temperature processing allows for more robust designs and also enables the construction of volume emitters and absorbers for lighting and solar cell applications.
Many applications experience high power densities. Whether a semiconducting device is generating the heat load or exposed to the heat load (i.e. concentrator solar cells), the materials used in the device must be able to withstand the operating conditions of the device. An example of this issue is the power LED. LEDs were initially restricted to low power applications like cell phones and indicators. As output levels have increased, the organic binders, adhesives, and encapsulants used in the past can no longer survive the operating conditions of the devices. The need therefore exists for methods, materials, and articles, which can withstand the high operating conditions, encountered in these new applications.
In addition, semiconducting devices, which experience high power densities, must provide a low thermal impedance path to the cooling means. Freestanding epitaxial chips offer the lowest thermal impedance path possible by eliminating unnecessary thermal boundary interfaces. Large area epitaxial chips are disclosed based on laser lifted gallium nitride layers. These freestanding epitaxial chips typically are greater than 20 microns thick and have an area of more than 1 cm2. These freestanding epitaxial chips are based on HYPE growths on sapphire and provide high crystal quality at low cost.
Freestanding epitaxial chips enable the use of processing temperature in excess of 700 degrees C. This processing temperature enables the use of brazing, welding, frets, low temperature glasses, and other high temperature processes not possible with more conventional fabrication techniques. A multitude of high temperature materials have been developed over the years, which can be used to create a variety of useful devices. In many cases, very simple devices interconnects are required as in LEDs, laser diodes, and power devices.
Unlike semiconductor processing of microprocessors, which require complex interconnects, a significant number of high resolution masking steps and vacuum processes, many simpler devices can be created using printing techniques for lower cost. However, high electrical conductivity materials typically require high processing temperatures to be robust. This invention discloses methods and articles that provide for low cost manufacture of high performance LEDs, power devices and solar cells, which can meet the requirements of high powered operation.